That said, no matter how reasonable it might be long-run, Batman won't kill (offer not void in space or on space men) and he doesn't use guns (offer not void on Batman Classic or on space men). While there are qualifications, Batman is best used when he has to fight these impulses. Even the super-gritty grandfather of gritty comics, “The Dark Knight Returns” features a Batman who just can't bring himself to kill The Joker.
Yeah, there are a lot of reasons not to kill The Joker. Most of them fiscal (recurring villains are easier to work with than trying to churn out new challenges every month and taking villains that buyers are interested in off of the table is a poor idea in general). Included in that principle of real-world finance is the transient nature of death in superhero comics (seriously, Cable didn't even get a funeral between his death in Cable and Deadpool and his appearance in Messiah Complex, with accompanying ongoing series. If I had gone to any amount of trouble to help kill Cable, I'd feel like a totally worthless limpdick, especially if killing him betrayed my principles. Gambit.); characters come back from the dead because they shouldn't have died in the first place. Death isn't even a time out some days, especially for The Joker, who quit needing rationales for his return for some time now. Good job everyone who ever killed The Joker before. You accomplished nothing and you're a murderer (oh, and Batman probably doesn't like you now, either). And remembering the justice issues mentioned earlier? Killing anyone (even The Joker) will come back to haunt you. If shooting The Hulk into space nearly split Earth in half, nothing good at all would come from killing The Joker.
Now, those are perfectly fine out-of-character reasons for Batman not to kill The Joker. The 'real' reason he doesn't do it is because Batman's a pretty pissed off guy in general. His desire to strike back against those who perpetuate chaos and injustice can't become a quest to eliminate everyone who transgresses. Redeeming and deterring criminals should be at least as important as stopping them from their current crimes. Shipping them off to Blackgate Penitentiary forever (forever pronounced “until they escape”) or until they're executed (providing they don't escape beforehand or get powers from being executed [Sinestro].) doesn't redeem anyone. Correctional facilities—in comics or in real life—are not for rehabilitation; they're punishment and places to lock criminals in with your Rorschachs. Anyway, if Batman didn't believe in reforming criminals, he'd be racking up a body count higher than Joker multiplied by Thanos.
If he were to judge which ones were redeemable and which ones weren't, then he becomes judge, jury, and executioner (not quite sure what the 'jury' portion of that is doing that isn't redundant with the 'judge' part, but he's Batman; I'm sure he can handle the collective responsibilities of three courtrooms, if need be). He would no longer serve the system; he would be a chaotic force of nature who's simply supplanting one form of imperfect order with another one. One based entirely on a single man's concept of right and wrong. He would also have to become jailer, judge, and reformist for every criminal who didn't need to be killed, since the resources available to the criminal justice system would no longer be available to him (although yes, if he can afford to buy The Justice League a new tower/satellite every time there's a cataclysmic event, then he can probably afford a massive secret prison under Wayne Manor).
Even if you cite The Joker as an exception to the rule, then you run into a strange situation. How many people do you have to kill to be the next Joker? How many do you have to be able to kill? How many do you have to be willing to kill? How many more lives can Batman save if he just starts profiling babies in his own version of Minority Report: Extreeeeeeme?
That baby was pure, concentrated evil.
After what point does Batman begin to decide that the bodies he's piling up are somehow more acceptable than those of anyone else? Because he's murdering people for a good cause? Batman can shoulder a lot of moral responsibility, but it's just too much to ask of him.
Speaking of responsibility and The Joker, it's been a part of his character for about twenty years now that The Joker only exists because of Batman. Without Batman, The Joker has no raison d'etre. The Joker isn't crazy in a world without Batman; the natural result of a man making himself a symbol to fight chaos is that someone specifically rises to stop him; to demonstrate that chaos is inevitable. Joker is the price that's paid for all of the other crime The Batman stops. Is that any reason to kill a man? Someone you made? Batman doesn't have to kill The Joker to keep him from killing; all he has to do is stop being Batman. Which one is more morally permissible? Who's to say that The Joker isn't a natural outgrowth? That he isn't a meme within Batman's universe in the same way that Batman is in ours? An idea that grows and changes with the times, but which infects the populace whenever one of their own carries himself too high? (That would be a badass villain for Batman, Inc.) Strike one down and another (or others) rise to take its place. Not only is Batman responsible for this man being The Joker, another one may very well rise to take his place.